The largest of the three skimmer species (4), the black skimmer is an unmistakable bird, best known for its remarkable beak (2) (5). As in all skimmers, this unusual structure is long and deep, with flattened, blade-like mandibles, and the lower mandible is longer than the upper (2) (3) (5) (6). The beak of the black skimmer is bright red at the base and black at the tip, and the upper mandible is slightly downcurved (2) (3) (5). Together with the bright red legs, it provides a colourful contrast to the white underparts and sides of the head, and the black upperparts and cap (3) (5) (6). The eye is somewhat invisible within the black feathering on the head (5), and the body appears unusually front-heavy due to the long beak and large head (2). The wings are long, narrow and pointed (5) (7), and the tail, which is white with a dark central streak, is slightly forked (2) (3) (5). The legs are short, with webbed toes (5) (7).

The male black skimmer is significantly larger than the female, with a longer beak, but is similar in colouration (2) (4) (5) (8). Non-breeding adults have a white collar on the neck, and somewhat browner upperparts, while immature birds are browner and more mottled than adults, with a duller beak (2) (3) (5). Three subspecies of black skimmer are recognised. Rynchops niger cinerascens and Rynchops niger intercedens are larger than Rynchops niger niger, and vary in the colour and markings on the wing linings and tail (2) (5) (6). The black skimmer has a relaxed and buoyant flight (2) (3) (5), and calls with an unusual, dog-like nasal “yip” (3) (5) (7).

The black skimmer occurs throughout the Americas, from the United States, through Mexico and Central America, and south into much of South America (9). R. n. niger is the most northerly subspecies, found along the Atlantic and Gulf coasts of the United States, south through Mexico and into Panama. A western population also breeds from southern California to Mexico (2) (5). R. n. cinerascens is found in northern South America and R. n. intercedens in southern South America (2) (3) (5). The species is migratory, with northern populations moving southwards during the winter, and southern populations sometimes wintering on the coast or moving to Central America or the Caribbean (2) (5) (8).

R. n. niger feeds mainly in coastal waters that are protected from open surf, such as in estuaries, bays, tidal pools, inlets and creeks (1). Nesting occurs on sandy beaches, sandbars, islands, shell banks, dredge islands and salt marsh, and the species may also occasionally be found on inland lakes (2) (5) (8) (1). The two southern subspecies nest mainly along sandbars and beaches of inland rivers, at times when water levels are lowest, and also use coastal beaches, lagoons, islands and estuaries (2) (3) (5). The black skimmer has been recorded at elevations of up to 3,800 metres on the shore of Lake Titicaca in Bolivia (2).

The structure of the black skimmer’s beak is related to its specialised foraging technique, which is unique to skimmers (2) (4). A feeding black skimmer flies low over water with the beak open and the lower mandible partially submerged, ‘skimming’ the water. If the lower mandible comes into contact with a prey item, such as a small fish or crustacean, the upper mandible snaps down while the head and neck double back under the body, securing the prey, which may be swallowed in flight or taken back to land (2) (5). The long beak and relatively long neck allow the skimmer to maintain its body position just above the water surface while skimming (2), and the beak can be opened unusually wide so that the upper mandible remains clear of the water. The knifelike edges of the beak help the bird to grasp slippery prey, and the neck muscles are very strong, enabling prey to be pulled from the water as the skimmer flies past (4). Although the black skimmer may sometimes wade, it does not swim or dive (2). Since skimming is a tactile rather than a visual hunting technique, skimmers are able to hunt at night, when many fish species come closer to the surface and strong daytime winds often lessen (2) (5) (7). Skimmers are the only birds in which the pupil of the eye constricts to a narrow vertical slit, an adaptation that may achieve a greater reduction in the pupil than with a circular opening, protecting the eyes from the bright glare of sunlight on water and sand during the day (2) (8) (10).

A social bird, the black skimmer roosts and breeds in colonies ranging in size from a few to thousands of pairs, and is often found in the company of gulls and terns, from which it may derive some protection from predators (2) (3) (5) (8) (1). Large, successful colonies usually occupy the same nest site from year to year (5) (1). The species is monogamous, and both the male and female help to prepare the nest and raise the chicks. The nest is a simple, shallow scrape in the sand, into which two to five eggs are laid, hatching after an incubation period of around 21 to 26 days (2) (5). Both the eggs and the chicks are white and are well camouflaged against the sand (2) (8). The black skimmer chick is quite well-developed and able to leave the nest after about a week, with fledging occurring after 28 to 30 days (2). The first attempts at skimming are made within about two days of the first flight, but initial success is low, and the young black skimmer may be dependent on the adults for a further few weeks (5). Interestingly, the two mandibles of the beak are of equal length on hatching. The lower mandible grows continuously faster than the upper, so that by the time the chick fledges it is already nearly 1 centimetre longer (2) (5). Its growth is kept in check by wear against the muddy or sandy bottom in shallow water, or by breakage on hitting obstructions, meaning that beak length and shape is quite variable between individuals and over time (2). The black skimmer is thought to breed from around the age of one to three years, and may live for up to at least 20 years in the wild (2) (5).

During the 19th Century, the black skimmer population was greatly reduced due to intense egg collection and hunting for feathers and food (2) (5) (8) (1). Although the species occurs over a large geographical range and currently has a relatively large global population (9), it still faces a number of threats. In the United States, flooding and human disturbance at breeding sites are major causes of colony failure, with sandy beaches increasingly being used for recreation and commercial development, or suffering disturbance from people, dogs and off-road vehicles (2) (5) (8) (1). Even slight disturbances can affect nesting success (8), reducing hatching and fledging and even causing adults to abandon the colony (5) (11). Despite being protected by law, the eggs are sometimes still taken, and in some areas the deliberate destruction of eggs and chicks and the shooting of adults is a problem (2) (5). Predators associated with humans, particularly dogs, cats and rats, may damage some colonies, and chemical pollution is also of concern, with organochlorines and heavy metals thought to accumulate in the eggs and feathers when the black skimmer feeds on contaminated fish (2). In South America, the rivers along which the black skimmer breeds are often the focus of human settlement, and increasing use of beach habitat by humans, as well as the collection of eggs and the possible depletion of fish stocks, poses a threat to many colonies (2) (3).

The black skimmer is quite adaptable in its use of feeding and nesting sites, often using man-made ponds and ditches, and breeding on gravel roofs and on areas of dredge deposition from oil exploration. However, the use of these artificial habitats results to a large extent from the black skimmer being displaced from natural beaches and salt marshes. Although adapted to endure natural disasters and heavy predation by readily changing colony site and re-nesting, the species is dependent on areas of open, calm water with a high concentration of fish, and so is vulnerable to any threats to fish populations, such as from oil spills or chemical pollution (2).

The future of the black skimmer is dependent on the protection of suitable breeding habitat, which is an ongoing problem as human populations expand and are increasingly attracted to beach areas, although even salt marsh colonies are vulnerable to predation and flood tides. Larger colonies tend to be more stable, and can be protected by preventing development at breeding sites, restricting access by off-road vehicles, erecting fences and educational signs, and wardening to prevent egg collection and to keep dogs out (5) (8). Annual monitoring of black skimmer populations, which can easily be done from the air to minimise disturbance, has been recommended in order to provide estimates of breeding numbers, and to identify sites needing protection. Monitoring and protection are also needed for fish stocks, to ensure that the quality and quantity are maintained and that any potential effects of pollutants in the food chain identified (5) (8). In South America, it is hoped that the preservation of large stretches of river and adjacent forest to protect local fishing industries will go some way to helping protect the populations of this remarkable bird (2).

Crustaceans: diverse group of arthropods (a phylum of animals with jointed limbs and a hard chitinous exoskeleton) characterised by the possession of two pairs of antennae, one pair of mandibles (parts of the mouthparts used for handling and processing food) and two pairs of maxillae (appendages used in eating, which are located behind the mandibles). Includes crabs, lobsters, shrimps, slaters, woodlice and barnacles.

Incubation: the act of incubating eggs, that is, keeping them warm so that development is possible.

Mandible: in birds, the lower jaw and beak, but the term is also used to denote the two parts of the beak.

Monogamous: having only one mate during a breeding season, or throughout the breeding life of a pair.

Subspecies: a population usually restricted to a geographical area that differs from other populations of the same species, but not to the extent of being classified as a separate species.